1. Field of Invention
The present invention relates generally to high-frequency waveguides and in particular to a frequency-sensitive attenuator for the dominant mode of a waveguide.
2. Discussion of the Prior Art
Practical waveguides are frequently designed so that they propagate electro-magnetic energy only in the dominant mode (TE.sub.10). These waveguides must be restricted in physical size in order to effect this mode exclusively. Accordingly, there are size constraints on the circuit elements that are utilized with dominant-mode waveguides.
The present invention deals specifically with waveguide frequency attenuators. It is frequently necessary to attenuate energy propagating through a waveguide in accordance with a controlled, frequency-attenuation characteristic. The prior art devices used to effect such controlled attenuation are large and bulky. For example, it is well known that Gunn Diode oscillators are subject to frequency jumping when subjected to high VSWR load reflections at frequencies associated with spurious frequency operation (usually 5-20 percent above the design frequency of the system). Since the loads in a microwave system are usually fairly well matched to the waveguide and oscillator at the design frequency, the dominant portion of the VSWR load reflections occur at 5-20 percent of the design frequency. A typical load VSWR curve for a center design frequency, F.sub.o, is shown in FIG. 1. Thus, when employing such an oscillator to drive a tuned load, a broadband isolator is usually required to keep the effective load VSWR below 3:1 over a 20 percent or more frequency band and thus stabilize the system. Such isolators are expensive, sensitive to external magnetic fields, and often take more room than is available in small, microwave packages.
The present invention utilizes the cutoff frequency parameter of a dominant-mode waveguide to produce a miniature, controllable, frequency attenuator. It is well known that each mode (the distinctive, spatial, field configuration of the electromagnetic energy) of transmission in a waveguide will carry energy through the waveguide only if the frequency of the energy is above a certain limiting or cutoff value for that particular mode. This value depends upon the size and configuration of the line as well as upon the particular mode of transmission. If these modes are excited in a waveguide at frequencies below their cutoff frequencies, they will carry no real energy down the waveguide, and the electric and magnetic field associated with any given mode will diminish exponentially with distance from the point of excitation.
In a rectangular waveguide, for any particular mode of transmission, the cutoff wavelength .lambda..sub.c is given in terms of the guide dimensions a and b by ##EQU1##
The dimensions a and b are shown in the cross-sectional view of the rectangular waveguide of FIG. 2. The terms m and n in the formula are the subscripts denoting the particular mode under consideration. m indicates the number of half-wave lengths in the transverse field intensity along the b dimension of the waveguide while n denotes the number of half-wave lengths in the a dimension. This formula holds for either the TE or TM modes of transmission.
The energy configuration for the dominant-mode of a rectangular waveguide (TE.sub.10), since it is the mode of primary interest, is shown in FIG. 2. The curve 30 represents this energy configuration.